Variable density recording optical system



Aug. 26, 1947. J. A. MAURER, JR

VARIABLE DENSITY RECORDING OPTICAL SYSTEM Original Filed Aug. 2, 1940 2Sheets-Sheet 1 F'IGJ IN! "ENTOR JOHN A. MAURER,JR.

/ AGENT Aug. 26, 1947.-

J; A. MAURER, JR VARIABLE DENSITY RECORDING OPTICAL SYSTEM originalFiled Aug. 2, 1540 2 Sheets-Sheet 2 1N1 ENTOR JOHN A. MAURERMQ. By IAGENT Patented Aug. 26, 1947 VARIABLE DENSITY, RECORDING OPTICAL SYSTEMJohn A. Maurer, Jr., J. A. Maui-er, Inc., tion of New York Originalapplication Au Divided and ber 21, 1944, Serial No.

8 Claims.

This invention relates to optical systems for the photographic recordingof electrical impulses on a moving film such as are used in soundrecording, picture transmission, and the like, and this application is adivision oi my application Serial No. 349,515, filed August 2, 1940,which is also assigned to J. A. Maurer, Inc.

More particularly, the invention relates to optical systems of the classreferred to above wherein a small mirror vibrated by an oscillographgalvanometer, or a similar device for translating electrical impulsesinto mechanical vibrations, modulates a light beam in accordance withthe electrical impulses to be recorded. The mirror oscillographrecording optical systems known heretofore, however, have thedisadvantage that the light flux from the recording light source, suchas the filament of an incandescent lamp, is not efilciently utilizedtherein. This unfavorable condition is due to the fact that the aperture01 the oscillograph mirror is the limiting aperture in the twocoordinate planes of the known optical systems, and that it cannot beenlarged beyond a certain degree since the physical size of the mirrormust be comparatively small in order to avoid distortions due to itsmass. For a given light source, therefore, the amount of light fluxreachposition in the optical system, for example, for selecting lightrays of a certain wave length, or for other purposes.

Another drawback of the known mirror oscillograph recording opticalsystems is that a portion of the light flux from the recording lightsource is not effectively prevented therein from falling on parts otherthan the oscillograph mirror, or on the structure housing the opticalsystem. This portion of the light flux is to some extent reflecteddiffusely, thus forming stray light even though the surfaces on which itis incident may be black. Such stray light is objectionable the movingfilm, to light flux modulated by the oscillograph mirror.

Still more particularly, the invention relates to mirror oscillographrecording optical systems by means of which the impulse records areproduced as tracks of constant width but varying density.

New York, N. Y., assignor to New York, N. Y., a corporagust 2, 1940,Serial No. this application Novem- Such tracks are known as variabledensity tracks. In the known variable density recording optical systemsof the mirror oscillograph type the variable density effect is obtainedby an imagery which is even less efllcient than the imagery describedhereinabove as being generally employed in the prior art mirroroscillograph recording optical systems. Alternatively, the variabledensity effect is obtained by the use of additional parts such as gradedscreens, penumbra stops, or the like. But it is difilcult to manufacturegraded screens which have a sufficiently accurate gradation, and the useof penumbra stops all but excludes certain modifications of the opticalsystem which are highly desirable from the standpoint 01' producingimpulse records with superior fidelity.

It is, therefore, the primary object of the present invention to providea variable density recording optical system of the mirror oscillographtype wherein the variable density effect is obtained solely by means ofan efficient imagery.

Another object of the invention is the provision of such an opticalsystem which is highly eflicient as regards the utilization of the lightflux from the recording light source.

Another object of the invention is the provision of such an opticalsystem whose limiting aperture in the one of its co-ordinate planes canbe made much larger than the aperture of the oscillograph mirror.

Another object of the invention is the provision of such an opticalsystem wherein the formation of stray light is reduced to a negligibleamount.

Another object of the invention is the provision oi such an opticalsystem wherein a linear relation may easily be established between theelectrical impulses to be recorded and the light transmission of thetrack used for their repro duction.

Another object of the invention is the provision of such an opticalsystem which is particularly lsatisfactory as regards ease ofmanufacture and convenience of adjustment.

Another object of the invention is the provision of such an opticalsystem which may be built with small physical size and at comparativelylittle cost.

Still other objects of the invention include those which are hereinafterstated or apparent, or which are incidental to the invention.

In the optical system according to the invention, the oscillographmirror is adapted to vibrate about a horizontal axis while the filmmoves past the recording point in a substantially vertical direction,the recording point being the point at which the optical axis of thesystem strikes the film. The optical system also has means for forming auniformly illuminated light spot whose horizontal extension varies in avertical direction, and means effecting an imagery of this light spotwhich is different in the two co-ordinate planes of the optical system:In the vertical plane, the light spot is conjugate to the recordingpoint while, in the horizontal plane, it is conjugate to a positionother than the recording point so that the recording point is conjugateto the light spot in only the vertical plane. By virtue of this imagery,a variable density effect is obtained at the recording point when theoscillograph mirror vibrates about its horizontal axis.

The imagery of the light spot is effected by a first set of imagingmeans with respect to which the light spot and the recording point areconjugate in,the vertical plane, and a second set of imaging means withrespect to which the light spot and said position are conjugate in thehorizontal plane. The first set of imaging means includes a firstimaging means which acts in the vertical plane, and a second imagingmeans which acts in only the vertical plane. The first imaging means isplaced in front of the oscillograph mirror and has one of its conjugatefoci at the light spot and the other substantially at a horizontal slitpositioned between the mirror and the recording point, and the secondimaging means has one of its conjugate tool at the slit and the other atthe recording point.

The second set of imaging means includes third and fourth imaging meansboth of which act in only the horizontal plane. The third imaging meanshas one of its conjugate foci at the light,

spot and the other substantially at the oscillograph mirror so that anintermediate image of the light spot is formed substantially on themirror. The fourth imaging means, in its turn, has one of its conjugatefeel at the intermediate image and the other at said position. When thenthe oscillograph mirror vibrates about its horizontal axis, a variabledensity track is produced on the film as it moves past the recordingpoint in a substantially vertical direction.

The means for forming the light spot, finally, include a recording lightsource and a screen with an opening whose horizontal extension varies ina vertical direction. The opening is uniformly illuminated by light fiuxfrom the light source, and its intermediate image is formedsubstantially on the oscillograph mirror as has been explained in thepreceding paragraph. At the same time, an image of the light source isformed substantially on the mirror by the action of a condenser lens inthe vertical plane. In this manner, substantially all the light fluxentering the optical system through the opening is controlled by theoscillograph mirror whereby the formation of stray light is reduced to anegligible amount.

In the foregoing brief explanation of the state of the art and summaryof the invention, and throughout the present specification, the termcoordinate planes designates two planes at right angles to each otherwhose line of inter section is the optical axis of the system. The

horizontal plane is the co-ordinate plane which contains the axis of theoscillograph mirror and the slit, while the vertical plane is theco-ordinate plane at right angles to the horizontal plane. The Q is theplane which conplane of the slit, finally, tains the slit, and is atright angles to both the vertical and,horizontal planes.

In the present specification, the terms vertical and horizontal thus arenot used in any absolute sense but merely in order to distinguishbetween two planes, or directions, at right angles to one another, andchoice between those terms has been determined solely by convenience indescription and illustration.

The invention will be better understood when the following descriptionis considered with the accompanying drawings of certain presentlypreferred embodiments thereof, and its scope will be pointed out in theappended claims.

In the drawings:

Fig. i is a diagrammatic perspective view of an optical system embodyingthe invention,

Fig. 2 is a diagrammatic longitudinal section in the vertical plane ofthe optical system shown in Fig. 1, the optical axis being representedas a straight line and an oscillograph mirror as an aperture,

Fig. 3 is a corresponding section in the horizontal plane,

Fig. 4 is an elevation of a part of the optical system of Figs. 1 to 3,

Figs. 5 to 8 show in elevation modifications of the part shown in Fig.4,

Figs. 9 and 10 are perspective views of modifications of two other partsof the optical system of Figs. 1 to 3,

Fig. 11 is a diagrammatic longitudinal section in the vertical plane ofa modification of the optical system of Figs. 1 to 3, and

Fig. 12 is a perspective view of a modification of still another part ofthe optical system of Figs. 1 to 3.

Referring first to Figs. 1 to 3, the novel variable density recordingoptical system has a light source such as the filament ID of anincandescent lamp II. A triangular opening IS in ascreen H is uniformlyilluminated by light flux from lamp filament III so that a uniformlyilluminated triangular light spot is formed at screen 14. The light beamdefined by lamp filament l0 and opening 13 proceeds through the opticalsystem and is deflected by the mirror ll of an oscillograph galvanometer(not shown) or similar device for translating electrical impulses intomechanical vibrations. It thus has a part which is incident from openingl3 upon mirror l1, and a part which is reflected from mirror l1 towardsthe recording point 21. Recording point 21 is the point at which theoptical axis of the system strikes the film 23, and film 23 moves pastrecording point 21 in a substantially vertical direction as indicated bythe arrow 29.

More particularly, opening I3 is an isosceles triangle whose baseextends horizontally, and mirror I1 is adapted to vibrate about an axisl8l8 which likewise extends horizontally. Furthermore, a horizontal slit2| is formed in a screen 22 which is placed between mirror 11 andrecording point 21.

A spherical condenser lens I2 is laced between lamp II and screen II,and a cylindrical lens l5 which has its cylinder axis vertical, isplaced between screen II and mirror ll. In front of mirror I] there isplaced a spherical lens l9 which acts on the reflected as well as theincident part oi. the light beam proceeding through the optical system.A second cylindrical lens 45 which also has its cylinder axis vertical,is placed between mirror l1 and screen 22, while a, third cylindricallens 4| has its cylinder axis horizontal and is placed between screen 22and recording point 21.

These five imaging means have focal lengths relative to the other partsof the optical system as follows (see Figs. 2 and 3) Spherical lens i2has one of its conjugate foci at lamp filament l0, and the othersubstantially at mirror l1, that is, either on mirror IT or at aposition close thereto. Cylindrical lens 15 has one of its conjugatefoci at opening l3, and the other substantially at mirror l1 so that anintermediate image of opening I3 is formed substantially on mirror I1.Spherical lens l9 has one of its conjugate foci at opening l3, and theother substantially at slit 2|. Cylindrical lens 45 has one of itsconjugate roci at the intermediate image of opening 13, and the other ata position B beyond recording point 21. Cylindrical lens 4| finally, hasone of its conjugate foci at slit 2|, and the other at recording point21.

By virtue of the arrangement described hereinabove of its various parts,the following imagery is performed in the optical system of Figs. 1 to3:

In the vertical plane (Fig. 2), spherical lens 19 forms an image of theuniformly illuminated opening I! in th plane of the horizontal slit 2i,or in a plane close to this plane. The image of opening i3 movesvertically across slit 2| when mirror 11 vibrates about the horizontalaxis 18- II. As much of slit 2| as is illuminated by the image ofopening i3, is imaged at recording point 21v by cylindrical lens 4|which is the sole means in the optical system for imaging slit 2| atrecording point 21. cording point 21 a horizontal line image of theilluminated portion of slit 2!. and this line image has sharp anddistinct horizontal boundaries. Likewise in the vertical plane,spherical lens 12 forms an image of lamp filament 10 substantially onmirror i1, thereby filling mirror l1 with light and also aiding in theuniform illumination of opening l3 by lamp filament Hi.

In the horizontal plane (Fig. 3), cylindrical lens i5 formssubstantially on mirror l1 the intermediate image of opening i3, and animage of the intermediate image is formed by cylindrical lens 45 atposition B. Since position B is beyond recording point 21, the lightflux acted upon by cylindrical lenses i5 and 45 arrives at recordingpoint 21 in a diffused condition so that it is spread out within thehorizontal boundaries of the line image formed at recording point 21 bythe action of cylindrical lens 4| in the vertical plane.

Lenses l2 and [9 are spherical and hence have power in the horizontal aswell as in the vertical plane. But their actions in thehorizontal planecan be disregarded for the following reasons:

On account of its position and relative focal length, spherical lens I2tends to image lamp filament l0 substantially on mirror [1 also in thehorizontal plane. The action, however, of cylindrical lens l5 interfereswith this imagery to such an extent that it becomes immaterial forattaining the objects of the present invention. On the other hand, thepower of spherical lens IS in the horizontal plane has no effect uponthe actions of cylindrical lenses l5 and 45 on account of the proximityof spherical lens i9 to mirror l1 which There thus is formed at reis, inthe horizonta plane, substantially at a common focus of cylindricallenses II and 45. No actions, therefore, of spherical lenses l2 and i9have been indicated in Fig. 3.

Cylindrical lenses l5 and 45, in their turn, do not interfere with theimagery in the vertical plane since they have their cylinder axesvertical, and hence act in only the horizontal plane. Correspondingly,cylindrical lens 4| does not interfere with the imagery in thehorizontal plane since it has its cylinder axis horizontal. and henceacts in only the vertical plane.

It will thus be seen that the imagery performed in the horizontal planeof the optical system of Figs. 1 to 3 differs from the imagery performedin its vertical plane in that recording point 21 is conjugate to openingII in only the latter plane. The effect of this difierence in imageryupon the line image formed at recording point 21 is as follows:

In the vertical plane, horizontal slit 2| is, with respect to sphericallens I9, conjugate to a horizontal line through opening II, for example,the broken line w-a shown in Fig. 4. Since, furthermore, recording point21 is in the vertical plane conjugate to slit 2i with respect tocylindrical lens 4|, it is also conJugate to line 0-4. The line image atrecording point 21 hence is an image of line a--aas far as its verticalextension, or width, is concerned, and it is, according to a well knownproperty of conjugates, made up of the light flux emanating from line0-4. Since, however, this light flux is, in the horizontal plane,diffused at recording point 21, the line image is not sharply defined atits ends. The light flux contained in the line image consequentlydecreases at its ends, but it is evenly distributed over its centralportion. This central portion thus is uniformly illuminated, and itsillumination is directly and linearly propor-- tional to the horizontalextension, or length, of line a--a. When, therefore, the horizontal linethrough opening I 3'15 short-as 18,!01 example, the broken line 11-!) inFig. 4-the central por-- tion of the line image is dim, while it isbright when that horizontal line is longas is. for example, the brokenline cc in Fig. 4. [The illuanimation of any horizontal line throughopening I3 is, of course, uniform since opening I 3 is uniformlyilluminated] The particular horizontal line through opening l3 to whichrecording point 21 is conjugate, is

determined by the angle of inclination of mirror I1. Normally, mirror 11is adjusted so that at its rest position, that is, when no electricalimpulses are applied to the oscillograph galvanometer on which it ismounted, recording point 21 is conjugate to line a-a, which line passesthrough opening I3 halfway between its tip and its base. When then theelectrical impulses to be recorded are applied in known manner to theoscillograph galvanometer, mirror l1 vibrates in accordance therewithabout the horizontal axis l8-l8 and in such a manner that, when theamplitude of its vibration is a maximum, recording point 21 is conjugateto line bb at the one extreme of its motion and to line 0-0 at the otherextreme thereof. The illumination of the central portion of the lineimage at recording point 21 hence varies in accordance with thevibration of mirror i1 and, therefore, the electrical impulses to berecorded. A variable density track 46 thus is produced on film 23 as itmoves past recording point 21.

But the vibration of mirror i1 varies also the horizontal extension, orlength, of the central portion of the line image, and when the minimumlength of the central portion is less than the desired horizontalextension, or width, of the variable density track 46, distortions areintroduced into track 46. In order to avoid these distortions, it isnecessary to control the minimum length of the central portion bysuitably choosing the focal length of cylindrical lens 45 and therebythe distance of position B from record ing point 21. Track 46 isundistorted, for example, when the ratio of the distance of position Bfrom recording point 21 to the distance of position B from cylindricallens 45 is at least as great as the ratio of the desired width of track46 to the aperture of cylindrical lens 45.

As has been explained hereinabove, the line image at recording point 21is not sharply defined at its ends. The edges of the variable densitytrack 46 hence are blurred and should be screened of! when prints aremade of track 46 so as to restrict the width of track 46 to its desiredvalue. Such screening ofl' now is commonly practiced in the printingfrom variable density tracks. If, however, it is desired to eliminatethe blurred edges when producing track 46 on film 23, two screens 46 and48 having vertical edges 50 and respectively, or similar means, may beemployed. As shown in Fig. 1, screens 46 and 49 are placed betweencylindrical lens 4| and recording point 21, and are spaced apart so thatedges 56 and 5i confine between themselves the desired minimum length ofthe central portion of the line image at recording point 21.

The illumination of the central portion of the line image at recordingpoint 21 varies in accordance with the vibration of mirror ll not onlywhen cylindrical lens 45 has one of its coniugate foci at position Bbut, quite in general, whenever a position other than recording point 21is, in the horizontal plane, conjugate to opening l3. Whenever thiscondition is fulfilled, the variable density track 46 is produced onfilm 23. For example, cylindrical lens 45 may have one of its conjugatefoci also at some position, including slit 2i, between lens 45 andrecording point 21, or it may have it at infinity, in which case thelight beam leaving lens 45 is parallel.

In all cases in which the position other than recording point 21,referred to in the preceding paragraph, is also a position other thanslit 2|- as is, for example, position B-the image of opening l3 formedsubstantially in the plane of silt 2| is a light spot of verticallyvarying illumination. When, however, slit 2! is conjugate to opening ISwith respect to cylindrical lenses l5 and 45, this image is a uniformlyilluminated triangular light spot. The formation of the latter lightspot in the plane of slit 2| facilitates the visual adjustment of theoptical system and also the visual monitoring of the recording done withit, but is otherwise immaterial for attaining the objects of theinvention.

The present invention thus provides a variable density recording opticalsystem of the mirror oscillograph type in which the variable densityeffect is obtained solely by means of a novel imagery and without theuse of additional parts such as graded screens, penumbra stops, or thelike. At the same time, the novel imagery brings it about that the lightflux from lamp filament III is utilized at recording point 21 in ahighly efficient manner, as will be seen from the followingconsiderations:

As has been pointed out hereinabove, the light flux from lamp filamentI0 is first employed for the uniform illumination of opening l3. In thehorizontal plane, which plane contains the hori zontal axis l6l6 ofmirror ll, an intermediate image of opening l3 then is formed bycylindrical lens l5 substantially on mirror i1, and this intermediateimage is imaged by cylindrical lens 45 at a position other thanrecording point 21, for example, position B. Mirror ll thus is in thehorizontal plane substantially at a common focus of cylindrical lensesl5 and 45. For any given angle of inclination of mirror l'l, therefore,the amount of light flux from opening i3 which arrives at recordingpoint 21, is limited in the horizontal plane by the aperture ofcylindrical lens 45 rather than the aperture of mirror II. Tocylindrical lens 45. however, there may be given an aperture which is asmuch as five times as great as the aperture which it is practical togive to mirror II.

This is a marked advance over the mirror oscillograph recording opticalsystems of the prior art wherein the light flux from the entranceposition corresponding to opening I! is diffused at the oscillographmirror in the two co-ordinate planes so that the mirror aperture is thelimiting aperture of the optical systems also in their horizontal plane.Since the physical size of the oscillograph mirror must be comparativelysmall in order to avoid distortions due to its mass, the above conditionhas been a serious obstacle to an eflicient utilization of the lightflux in those optical systems. The advantage gained in this respect bythe optical system of Figs. 1 to 3 is considerable because, as is wellknown to those skilled in the art, the efficiency with which the lightflux from a given light source is utilized in an optical system, isapproximately proportional to the product of the limiting apertures inits two co-ordinate planes.

Another advantage of having mirror l1 substantially at a common focus oftwo imaging means which act in only the horizontal plane, is that smalldeviations of mirror l'I about a vertical axis have a negligible effecton the imagery in the horizontal plane. Mirror Il need therefore beaccurately adjusted only about the horizontal axis l8-I6. This greatlyincreases the ease of adjustment of the optical system, and isparticularly important when it is necessary to replace the oscillographgalvanometer on which mirror I1 is mounted.

A further advantage of the novel imagery embodied, by way of example, inthe optical system of Figs, 1 to 3 resides in the fact that there isformed substantially on mirror I! an image of lamp filament ill by theaction of spherical lens 12 in the vertical plane and, simultaneously,the intermediate image of opening l3 by the action of cylindrical lensl5 in the horizontal plane. It thus is possible so to control the lightflux which enters the optical system through opening l3 that it is allincident within the working aperture of mirror ll. This result is bestobtained when the focal length of spherical lens l2 and the position oflamp II are chosen so that the image of lamp filament Ill has a verticaldimension no larger than that of mirror I1, and when the focal length ofcylindrical lens l5 and the position of screen M are chosen so that thelargest horizontal dimension of the intermediate image is no larger thanthe horizontal dimension of mirror ll. If these conditions arefulfilled, all the light flux passing through opening I3 is subject tocontrol by mirror ll, whereby the formation of stray light in theoptical system is reduced to a negligible amount.

The employment, finally, of cylindrical lens M in the portion of theoptical system between screen 22 and recording point 27 has certaininherent advantages: Cylindrical lens 4| may have a short focal lengthso that the optical system may be built with small physical size.Moreover, a cylindrical lens of short focal length is less expensivethan a spherical lens system well enough corrected to be capable offorming, over the same length, an equally sharp line image. The opticalsystem of Figs. 1 to 3 may hence be built with greater compactness andat less cost than the mirroroscillograph recording optical systems knownheretofore.

The optical system shown in Figs. 1 to 3 as an embodiment of the imageryaccording to the in- "l'tlm'l may be modified, without affecting thebasic principles of its operation, as, follows:

(1) Opening iii in screen i4 is shown in Figs. It and l and has beendescribed hereinabove, as being an isosceles triangle whose base extendsI: izontally. However, any other opening whose Ii. orizontal extensionvaries in a vertical direction, be substituted for opening iii toproduce the iable density track ib on film 23. For examthe opening inscreen M may be a right-am triangle with one of the sides adjacent tolight angle extending horizontally as is the l hing till shown in Fig.5, or there may one more sawtooth projections extending into it as theydo the openings iii and 552 shown in Figs. 6 and ely.

drugs ill, and c more inclined edges which are straight, chat theirhorizontal extension varies in a 1111 manner. But the horizontalextension of the opening in screen bl may also be varied in non-linearmanner, for example, in order to es tablish a linear relation betweenthe electrical. impulses to be recorded in. track 16 and the lighttransmission of the track used for their reprc duction. This resultobtained when one or more oi the inclined edges or" the opening inscreen i l are curved as are the two inclined edges of the opening ilshown in Fig. 8, a method of computing suitable curvatures beingdisclosed,

us. distort 10l'l-lll16El,l characteristic of the oscillogra', lvanometer on which mirror ii is mounted, or or the emulsions or" therecording and printing films, or of both.

li'i 'hen any of the openings is in screen i4 and receives 1p filamentiii through spherical lens there is formed at screen i l a uniformlyilluminated i at spot whose horizontal eimension varies ll direction.Since, furthermore, th his from this light spot is, in the hori plane,diifused at recording point ii, a single '1 1e image is formed at thispoint in the case of all openings. When, therefore, mirror ii vibratesabout the horizontal axis Iii-l 8, the illumination of the centralportion of the line image varies in linear relation to the variation inhorizontal tension of the light spot at screen l4. Since, finally, thelatter variation i effected only by the inclined edge, or edges, of theopeningin screen M, the lower portion of screen M may be omitteddesired, as indicated in Fig. 5 by the broken line e--e, for example.

its in Fig. 4, the broken line a-a. indicates also are bounded by inFigs. 5 to 8 the horizontal line through the opening in screen M whichis normally conjugate to recording point 21 when mirror I! is at rest.

(2) Condenser lens I2 is shown in Figs. 1 to 3, and ha been describedhereinabove, as being spherical. It hence acts in both the vertical andhorizontal planes. However, as has been pointed out hereinabove, itsaction in the horizontal plane is immaterial as far as the novel imagerydisclosed in this specification is concerned. Spherical condenser lensi2 may, therefore, be replaced by a cylindrical condenser lens 65 whichhas its cylinder axis horizontal and hence acts in only the verticalplane. Like spherical lens l2, cylindrical lens 65 has one of itsconjugate foci at lamp fila= ment ill, and the other substantially atmirror ll. Cylindrical lens may, furthermore, have the same position asspherical lens 12, in which position it is shown in Fig. 9. But since itacts in only the vertical plane, it may also have any other positionbetween lamp ii and mirror H which is consistent with its function toimage lamp filament iii substantially on mirror H.

In designing an actual optical system with cylindrical condenser lenllii, however, the horizontal extension of lamp filament ill should bemade so great that the opening in screen id, as seen from cylindricallens iii, is completely filled with light.

(3) It has been explained hereinabove that, while spherical lens l9 haspower in both the vertical and horizontal planes, its action in thehorizontal plane can be disregarded. Spherical lens ill may therefore bereplaced by a cylindri- 'cal lens lit which has its cylinder aXishorizontal. and hence acts in only the vertical plane. Like sphericallens ill, cylindrical lens 66 has one of its conjugate feel at theopening in screen M, and the other substantially at slit 2i. Cylindricallens at may, furthermore, have the same position as spherical lens [9,in which position it is shown in. Fig. 10. But since it acts in only thevertical plane, it may have any other position between screens Hi and 22which is consistout with its function to image the opening in screen itsubstantially in the plane of slit 2|. Spherical. lens it, on the otherhand, should be ci mirror ii, as shown in Fig. l, lest Wltll the imageryin the horizontal 'i i u the light beam defined by lamp filabe openingin screen i4 is incident l at a sufficiently small angle, a :"li lens ii may be substituted for the two cylindrical lenses i5 and 45. Likecylindrical lenses iii and d5, cylindrical lens H has its cylinder axisvertical, and it is placed so as to be transversed, by the reflected aswell as the incident part of the light beam proceeding through theoptical system. The relative focal length of cylindrical lens ii is sochosen that the opening in screen it and a position on, or close to,mirror ill are conjugate with respect to cy lindrical lens ii on theincident part, and this position and position B, or an equivalentposition, are conjugate with respect to cylindrical lens ii on the.reflected part of the light beam. this manner, cylindrical lens l'iforms the intermediate image of the opening in screen it substantiallymirror ii and, simultaneously, ima es the intermediate image at aposition other than point The angle which the light beam is incidentupon mirror l1, may be made sufficiently small by considerablylengthening the optical system mechanically. However, this end may beaccomplished in a more convenient way which, at the same time, providesfor a very compact mechanical design of the optical system and which isshown, by way Of example, in Fig. 11. It consists of placing areflecting prism 10 between screen it and mirror i1 whereby the lightbeam is folded so that it is incident upon mirror I! at a small angleand cylindrical lens II is traversed by both the incident and reflectedparts Of the light beam. In place of prism I there may be employed othersuitable beam folding means such as mirrors, or the like.

(5) Whenever it is desired to employ for the imagery in the horizontalplane of the optical system two lenses instead of the single cylindricallens 1i, cylindrical lens 45 may be replaced by a spherical lens which,however, must be placed adjacent to screen 22 as shown in Fig, 12. Whenspherical lens 15 is so placed, its action in the vertical plane isbarred by screen 22 so that it acts in only the horizontal plane.Spherical lens 15 may be placed on either side of screen 22, but must beclose thereto in both cases lest it interfere with the imagery in thevertical plane.

Like cylindrical lens 45, spherical lens 15 has one of its conjugatefoci at the intermediate image of the opening in screen I4, and theother at a position other than recording point 21. The substitution ofspherical lens 15 for cylindrical lens has the advantage that aspherical lens at screen 22 is cheaper, and easier to adjust, than acylindrical lens. 45, On the other hand, has the advantage that itsposition can be chosen independently of the position of screen 22,whereby the design of the optical system is facilitated.

(6) If it is desired to employ the optical system for recording sound inaccordance with the method generally known as "noiseless recording, thewell known ground noise reduction systems may be used in conjunctiontherewith, a will easily be understood by those skilled in the art.

Many other modifications of the invention will readily suggestthemselves to those skilled in the art. The invention, therefore, is notto be limited, except in so far as is necessitated by the prior art andby the spirit of the appended claims.

What is claimed is:

i. In an optical system, the combination of means for forming auniformly illuminated light spot whose horizontal extension varies in avertical direction; a mirror adapted to vibrate about a horizontal axis;means forming a slit which extends horizontally; a recording point pastwhich a film may move in a substantially vertical direction; and meansfor forming at said recording point a horizontal line image whoseillumination varies when said mirror vibrates about said axis; said lastmentioned means including first imaging means placed between said lightspot and said mirror, and having first and second conjugate foci; secondimaging means placed between said mirror and said slit formin means, andhaving third and fourth conjugate foci; third imaging means placed infront of said mirror, and having fifth and sixth conjugate fool; andfourth imaging means placed between said slit forming means and saidrecording point. and having seventh and eighth conjugate foci: saidfirst imaging means acting in only the horizontal plane and having saidfirst focus at said The use of cylindrical lens light spot, and saidsecond focus substantially at said mirror so that an intermediate imageof said light spot is formed substantially on said mirror; said secondimaging means acting in only the horizontal plane and having said thirdfocus at said intermediate image, and said fourth focus at a positionother than said recording point; said third imaging means acting in thevertical plane and having said fifth focus at said light spot, and saidsixth focus substantially at said slit; and said fourth imaging meansacting in only the vertical plane and having said seventh focus at saidslit, and said eighth focus at said recording point.

2. The combination defined in claim 1 wherein said first imaging meansis a cylindrical lens having its cylinder axis vertical.

3. The combination defined in claim 1 wherein said first and secondimaging means are each a cylindrical lens having its cylinder axisvertical.

4. The combination defined in claim 1 wherein said second imaging meansis a spherical lens placed adjacent to said slit forming means.

5. The combination defined in claim 1 wherein said third imaging meansis a spherical lens, and said fourth imaging means is a cylindrical lenshaving its cyliinder axis horizontal.

6. The combination defined in claim 1 wherein said third and fourthimaging means are each a cylindrical lens having its cylinder axishorizontal.

'I. The combination defined in claim 1 wherein said first and secondimaging means are each a cylindrical lens having its cylinder axisvertical, said third imaging means is a spherical lens, and said fourthimaging means is a cylindrical lens having its cylinder axis horizontal.

8. A variable density recording optical system including in combination,a light source; a screen with an opening whose horizontal extensionvaries in a vertical direction, said opening being uniformly illuminatedby said light source; a mirror adapted to vibrate about a horizontalaxis; means forming a slit which extends horizontally; a "recordingpoint past which a film may move in a substantially vertical direction;a first spherical lens placed between said light source and said screen,and having first and second conjugate foci; a first cylindrical lensplaced between said screen and said mirror, and having third and fourthconjugate feel; a second cylindrical lens placed between said mirror andsaid slit forming. means, and having fifth and sixth conjugate foci; asecond spherical lens placed in front of said mirror, and having seventhand eighth conjugate foci; and a third cylindrical lens placed betweensaid slit forming means and said recording point, and having ninth andtenth conjugate foci: said first spherical lens having said first focusat said light source, and said second focus substantially at saidmirror; said first cylindrical lens having its cylinder axis verticaland having said third focus at said opening, and said fourth focussubstantially at said mirror so that an intermediate image of saidopening is formed substantially on said mirror; said second cylindricallens having its cylinder axis vertical and having said fifth focus atsaid intermediate image, and said sixth focus at a position beyond saidrecording point; said second spherical lens having said seventh focus atsaid opening, and said eighth focus substantially at said slit; and saidthird cylin- 13 drical lens having its cylinder axis horizontal andhaving said ninth focus at said slit, and mid tenth focus at saidrecording point.

JOHN A. MAURER, JR

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS 10 Name Date Dimmick Sept. 19, 1939 Number NumberNumber Name Date Dimmick May 9, 1939 Dimmick Apr. 30, 1935 Cook Aug. 9,1938 Dimmick Aug. 25, I936 McLeod et a1. Sept. 16, 1941 Emmerich Apr. 7,1936 FOREIGN PATENTS Country Date Great Britain Feb. 5, 1935

